A ball screw device, in general, comprises a screw shaft having a thread groove in the outer peripheral surface thereof, a nut having a thread groove in the inner peripheral surface thereof and externally mounted on the screw shaft, and a multiplicity of balls interposed between the respective thread grooves of the screw shaft and the nut. The balls, in accordance with the rotation of the nut or the screw shaft, circulate between the respective thread grooves, which thereby causes a relative telescopic motion between the nut and the screw shaft. The balls, due to the circulation thereof, require preventive measures not to slip out of between the respective thread grooves. As a ball circulation mechanism capable of preventing the slip-out of balls, a ball screw device having an externally mounted part for circulating balls, that is a so-called deflector, is available. The deflector is described below.
The deflector, in general, serves to couple the upstream side of the thread groove, which balls are rolled from, and the downstream side thereof, which the balls are rolled to, so that the balls on the downstream side of the thread groove are returned to the upstream side thereof over a ridge. The deflector is, being fitted into a radial through hole formed on a nut, fixed to the through hole by an adhesive. The deflector has, in the inner diameter surface thereof, a ball circulation groove for returning the balls, over the ridge, to the upstream side from the downstream side in substantially one turn of the thread grooves over the ridge.
Such a ball screw device requires an externally mounted part, that is the deflector, which generates additional costs for the deflector, processing the through hole for mounting the deflector to a nut and mounting the deflector to the nut. Besides, when the deflector is mounted to the through hole of the nut rather imprecisely, the ball circulation groove and thread groove cannot be precisely positioned, which results in a degraded circulation performance of the balls. Thus, mounting the deflector requires precision, which is costly. For these reasons, it has been desirable to develop a ball screw device capable of circulating balls smoothly without using the deflector.
Therefore, a main object of the present invention is to provide a ball screw device capable of circulating balls without using a deflector.